In order to realize the ubiquitous society in the future, and as small portable appliances prevail, the demand for nonvolatile memory, which is small, has a large capacity, and is capable of reading and writing at a high speed and operating with a low power consumption, is increasing.
A magnetic random access memory (MRAM) using ferromagnetic tunnel junction showing a tunneling magnetoresistive (TMR) effect is drawing the attention as a next-generation solid-state nonvolatile memory. An MTJ (Magnetic Tunnel Junction) element with ferromagnetic tunnel junction has a stacked structure formed on a substrate, on which transistors and wirings are formed, the stacked structure including a first ferromagnetic layer (storage layer), in which the magnetization direction can be changed, a nonmagnetic layer (insulating tunnel barrier layer) and a second ferromagnetic layer (magnetization fixed layer), in which a predetermined magnetization direction is fixed.
When a current is caused to flow through a ferromagnetic tunnel junction, the current flows to tunnel through the tunnel barrier layer. In a magnetic memory device including an MTJ element with ferromagnetic tunnel junction as a memory cell, at least one ferromagnetic layer is assumed to be a reference layer, in which the magnetization direction is fixed, and another ferromagnetic layer is assumed to be a storage layer. In such a cell, it is possible to store information by assigning “0” or “1” of binary data to the parallel or antiparallel state of the magnetizations of the reference layer (magnetization fixed layer) and the storage layer.
The reading of information is performed by flowing a current to tunnel through the tunnel barrier layer and detecting the resistance value. In such a case, the resistance value of the MTJ element changes in accordance with the cosine of the relative angle between the magnetizations of the first and the second ferromagnetic layers. Furthermore, the junction resistance value is the lowest when the directions of magnetizations in the first and the second ferromagnetic layers are in a parallel state (the same direction), and the highest when they are in an antiparallel state (opposite directions). This change in resistance is called “tunneling magnetoresistive effect (TMR effect).”
As a method of recording of information to the storage layer, a method is suggested in which the MT) element is directly energized so that the magnetization of the storage layer is switched by the spin torque transfer from the reference layer (spin torque transfer switching method). In the spin torque transfer switching method, as the size of the memory cell decreases, the amount of current required for the writing decreases. Thus, it is possible to increase the capacity easily. The reading of information from the memory cell is performed by flowing a current through the ferromagnetic tunnel junction, and detecting a change in resistance caused by the TMR effect. A magnetic memory is formed by arranging a number of such memory cells.
The write efficiency of an MTJ element, in which the first and the second ferromagnetic layers have magnetizations that are parallel to the film plane, is not good. Therefore, it is difficult to increase the capacity of such an element to a giga-bit level. In contrast, the write efficiency of an MTJ element (also called “perpendicular magnetization MTJ element), in which the first and the second ferromagnetic layers have magnetizations that are perpendicular to the film plane, is good. Thus, it is possible to obtain a large capacity.
A conventional perpendicular magnetization MTJ element has been formed by forming a magnetic body with a polycrystalline structure or amorphous structure directly on transistors by sputtering. As a result, the in-plane orientation of each of the first ferromagnetic layer and the second ferromagnetic layer differs for each crystal grain, resulting in that the magnetic interaction between the crystal grains is weakened. For this reason, it is not possible to obtain a film characteristic of a next-generation MRAM of a giga bit level. This has a great influence on variations in perpendicular magnetic property, etc.